Progression elevated gene-3 (PEG-3) was cloned from a tumor progression model based on rat embryo cells (Babiss et al., 1985, Science 228: 1099-1101; Fisher et al., 1978, Proc Natl Acad Sci USA 75: 2311-2314; Su et al., 1997, Proc Natl Acad Sci USA 94: 9125-9130). E11 is a mutant adenovirus type 5 (H5ts125)-transformed rat embryo cell clone that forms small, slow-growing and compact tumors. E11-NMT is a clone of E11 that has been selected for aggressiveness by passage through a nude mouse and forms rapidly growing, highly aggressive tumors (Babiss et al., 1985, Science 228: 1099-1101). Subtraction hybridization of an E11 cDNA library from an E11-NMT cDNA library identified PEG-3 (Su et al., 1997, Proc Natl Acad Sci USA 94: 9125-9130). Elevated PEG-3 expression has been documented in E11-NMT cells in comparison with E11 cells and also in normal cloned rat embryo fibroblast (CREF) cells displaying a transformed/tumorigenic phenotype, as a consequence of expression of diverse acting oncogenes, including Ha-ras, v-src, human papilloma virus type-18-transforming genes and a specific mutant of Ad5 (H5hr1), relative to parental CREF cells (Su et al., 1997, Proc Natl Acad Sci USA 94: 9125-9130). Ectopic expression of PEG-3 in E11 cells markedly augments in vitro anchorage independent growth and increases their oncogenic potential in nude mice as reflected by a shorter tumor latency time and the production of larger tumors with increased vascularization (Su et al., 1997, Proc Natl Acad Sci USA 94: 9125-9130; Su et al., 1999, Proc Natl Acad Sci USA 96: 15115-15120). As a corollary, E11-NMT cells stably expressing antisense PEG-3 lose their progressed cancer phenotype (Su et al., 1999, Proc Natl Acad Sci USA 96: 15115-15120). Overexpression of PEG-3 induces genomic instability, modulates the expression of important genes involved in centrosomal duplication and augments the invasive capability by increasing matrix metalloproteinase activity indicating that PEG-3 facilitates tumor progression by multiple pathways (Su et al., 2002, J Cell Physiol 192: 34-44).
When it was cloned, PEG-3 was observed to have high homology to hamster GADD34, except that it lacked the unique carboxy-terminal domain of GADD34 that is conserved in all species (Su et al., 1997, Proc. Natl. Acad. Sci. U.S.A. 94:125-130). GADD34 is a member of the Growth Arrest and DNA Damage (“GADD”) inducible family of genes (Fornace et al., 1989, Mol. Cell Bio. 9:4196-4203; Takekawa et al., 1998, Cell 95:521-530; Zhan et al., 1994, Mol. Cell Biol. 14:2361-2371). GADD34 was first identified in hamsters (Fornace et al., 1989, Mol. Cell Bio. 9:4196-4203), and subsequently has been identified in human (Hollander et al., 1997, J. Biol. Chem. 272:13731-13737), mouse (as Myd116; Lord et al., 1990. Nucl. Acids Res. 18:2823) and rat; it exhibits high homology and similar domain structure across species. Induction of GADD34 has been shown to be associated with induction of apoptosis by diverse mechanisms and overexpression of GADD34 alone can markedly inhibit cell growth as a consequence of apoptosis (Lord et al., 1990. Nucl. Acids Res. 18:2823; Grishin et al., 2001, Proc. Natl. Acad. Sci. U.S.A. 98:10172-10177; Hollander et al., 2003, Oncogene 22:3827-3832; Hollander et al., 2001, Int. J. Cancer 96:22-31; Hollander et al., 1997, J. Biol. Chem. 272:13731-13737). The amino-terminal third of the GADD34 protein is involved in augmentation of apoptosis following ionizing radiation (Lord et al., 1990. Nucl. Acids Res. 18:2823). In human, mouse, hamster and rat, GADD34 protein contains 674, 657, 590 and 577 amino acids, respectively (Fornace et al., 1989, Mol. Cell Bio. 9:4196-4203; Hollander et al., 2003, Oncogene 22:3827-3832; Hollander et al., 1997, J. Biol. Chem. 272:13731-13737; Lord et al., 1990. Nucl. Acids Res. 18:2823; Zhan et al., 1994, Mol. Cell Biol. 14:2361-2371). GADD34 protein contains a series of repeated sequences in the central region. These repeats are 34, 40 and 39 amino acids long for human, mouse, hamster and rat GADD34 proteins, respectively (Bulavin et al., 1999, EMBO J. 18:6845-6854; Chou et al., 1994, Proc. Natl. Acad. Sci. U.S.A. 91:5247-5251). The human protein has four copies of the repeat sequence, while the mouse, hamster and rat protein has 4.5, 3.5 and 2.5 copies, respectively (Hollander et al., 2003, Oncogene 22:3827-3832; Hollander et al., 1997, J. Biol. Chem. 272:13731-13737). In rodents the repeat sequences are arranged in tandem, while in humans these repeats are separated by varying numbers of amino acids. The functional significance of these repeats is unclear. Approximately 90 amino acids at the carboxy-terminal end of GADD34 protein are highly conserved across species, with approximately 90 percent amino acid homology.
Recent cloning of rat GADD34 revealed that PEG-3 is identical to rat GADD34 in the first 415 amino acids (Hollander et al., 2003, Oncogene 22:3827-3832; Su et al., 1997, Proc. Natl. Acad. Sci. U.S.A. 95:14400-14405). A single base deletion resulted in frame-shift and premature appearance of stop codon resulting in C-terminal truncation and sequence divergence of PEG-3 from rat GADD34 (Su et al., 1997, Proc. Natl. Acad. Sci. U.S.A. 95:14400-14405). Southern blot analysis showed that there is a single locus for GADD34 in the normal rat genome suggesting, that PEG-3, a mutated form of GADD34, might be generated during the process of transformation and tumor progression (Hollander et al., 2003, Oncogene 22:3827-3832).